Field of the Invention
The present invention relates to a rotating electric machine, and, more particularly, to an axial gap type rotating electric machine used as an electric motor and a power generator.
Description of the Related Art
Reduction in weight, thickness, length, and size of rotating electric machines used as electric motors and power generators is strongly required in the market. In recent years, improvement in energy saving and high efficiency of rotating electric machines is also increasingly required in order to address global warming. Reduction in vibration, noise, and cost of rotating electric machines is also strongly required. Under the circumstances, an axial gap type rotating electric machine having an air gap in a rotating shaft direction has a flattened shape, which is advantageous for reduction in thickness. Further, if a rotor of the axial gap type rotating electric machine is formed in a discoid shape, inertia thereof can be reduced, and hence the axial gap type rotating electric machine is suitable for a constant speed operation and a variable speed operation. Consequently, the axial gap type rotating electric machine starts to attract attention in recent years, and various modes are known therefor.
Japanese Patent Laid-Open No. 2012-130086 is proposed as a related conventional art.
Rotating electric machines are categorized into a radial gap type and an axial gap type, and rotation principles of the two types are the same as each other.
A brushless DC motor (hereinafter, referred to as BLDCM) and a synchronous power generator, in which a permanent magnet is used for a rotor, or a switched reluctance motor (hereinafter, referred to as SRM), in which a permanent magnet is not used for a rotor and teeth of a magnetic material are provided instead, are used as conventional general radial gap type rotating electric machines. According to an art for the BLDCM and the synchronous power generator or the SRM, a stator iron core is formed by laminating silicon steel plates, and, in a case of placing importance on an inexpensive price and efficiency, a winding wire is generally wound in a concentrated manner.
If a winding wire is wound in a distributed manner, a coil end portion that does not contribute to torque generation becomes large, a copper loss increases, and efficiency decreases. In comparison, if a winding wire is wound in a concentrated manner, the winding wire is simple and can be wound directly in a slot, so that the winding wire can be inexpensive.
In recent years, axial gap type BLDCM and SRM are also studied as in-vehicle motors for driving hybrid cars and electric cars. This is because flattened shapes of these motors are convenient in a case where these motors are provided together with an engine or are configured as in-wheel motors. It is known that, particularly for the axial gap type BLDC motor, field strengthening control is performed at the time of start-up and low-speed rotation in order to obtain a high torque, whereas field weakening control is performed at the time of high-speed rotation in order to obtain high-speed rotation. A reason for performing such field control is as follows: at the time of a low speed, if a field system magnetic flux is large, a high torque is obtained; but, at the time of a high speed, if the field system magnetic flux is large, an electromotive force constant is also large, a motor internal induced voltage approaches a power supply voltage, and this prevents current from flowing and makes the torque lower. For this reason, at the time of high-speed rotation, the field system magnetic flux is generated in a direction opposite to a direction in which a rotor magnetic pole is magnetized, and the torque at the time of the high speed is increased by the field weakening. In order to avoid this, it is conceivable to perform field control using a multipolar permanent magnet field motor, but such control using the multipolar permanent magnet field motor is complicated and expensive because, for example, a vector control technique needs to be effectively utilized. In this regard, in a case of the axial gap type BLDCM and SRM, if the rotor is moved in an axial direction such that a distance that is an air gap between a stator and a rotor becomes shorter at the time of low-speed rotation and becomes longer at the time of high-speed rotation, characteristics similar to those obtained by control for strengthening or weakening the field system magnetic flux can be produced.
FIG. 10 is a cross sectional view including a shaft of an axial gap type BLDCM according to a typical conventional art. FIG. 11 is a cross sectional view indicated by arrows D-D in FIG. 10. In FIG. 10 and FIG. 11, a function of varying an air gap between a rotor and a stator is not particularly provided, and a conventional typical structure is described. In this example, the number of portions of a stator iron core 19 is six, a winding wire 2 has three phases, and a rotor is tetrapolar. The winding wire 2 is wound around the stator iron core 19, and the stator iron core 19 is connected to a power supply by a lead wire for power supply 3. Illustration of a Hall element and the like is omitted. The rotor includes a permanent magnet 5. The permanent magnet 5 includes four fan-shaped segment magnets magnetized in an axial direction. Opposite polarities of the segment magnets are alternately arranged in a circumferential direction. The permanent magnet 5 is planarly opposed to the stator iron core 19 with the intermediation of an air gap in the axial direction. That is, this structure is of plane air gap type. A back yoke 17 fixedly attached to a rotating shaft 16 forms a magnetic circuit. That is, a combination of the permanent magnet 5, the back yoke 17, and the rotating shaft 16 is the rotor, and is rotatably supported by a housing 18 and the stator iron core 19 with the intermediation of a bearing 11.
Japanese Patent Laid-Open No. 2012-130086 is known as a conventional art for further forcibly varying a gap length in the typical axial gap type BLDC motor by means of an external force. According to Japanese Patent Laid-Open No. 2012-130086, a rotor is moved in an axial direction by a variable gap mechanism that is operated by a power source different from a rotational force of the axial gap type rotating electric machine, whereby an air gap width can be changed.
Unfortunately, conventional axial gap type rotating electric machines including the machine illustrated in FIG. 10 and FIG. 11 are of plane air gap type in which a field magnet and a stator iron core are planarly opposed to each other. Hence, compared with radial gap type motors, a torque cannot be made higher for the reason that a minimum air gap cannot be made small and other reasons, and practicality of such conventional axial gap type rotating electric machines is not sufficient in actual use. Further, compared with the radial gap type, an air gap length in the axial gap type needs to be approximately twice larger in consideration of rotor plane deflection, so that efficiency decreases accordingly. Furthermore, air gap length-to-torque characteristics do not linearly change but non-linearly change, and hence controllability is not favorable.
The present invention, which has been made in view of the above-mentioned problems, has an object to provide an inexpensive high-performance rotating electric machine having practicality even in a case of a high output and also having high efficiency and high controllability.